The DNA Double-Strand Break Repair Gene hMRE11 Is Mutated in Individuals with an Ataxia-Telangiectasia-like Disorder

Abstract

We show that hypomorphic mutations in hMRE11, but not in ATM, are present in certain individuals with an ataxia-telangiectasia-like disorder (ATLD). The cellular features resulting from these hMRE11 mutations are similar to those seen in A-T as well as NBS and include hypersensitivity to ionizing radiation, radioresistant DNA synthesis, and abrogation of ATM-dependent events, such as the activation of Jun kinase following exposure to gamma irradiation. Although the mutant hMre11 proteins retain some ability to interact with hRad50 and Nbs1, formation of ionizing radiation-induced hMre11 and Nbs1 foci was absent in hMRE11 mutant cells. These data demonstrate that ATM and the hMre11/hRad50/Nbs1 protein complex act in the same DNA damage response pathway and link hMre11 to the complex pathology of A-T.

ERK activity facilitates activation of the S-phase DNA damage checkpoint by modulating ATR function

“…As cells were treated with subapoptotic doses of HU, this sensitization of HU toxicity is most likely attributable to a cellular catastrophe that is caused by DNA damage when checkpoint function is compromised. Our observation is similar to the enhanced toxicity of AT cells (cells without ATM function or with compromised ATM function) to genotoxic stresses (Shiloh, 1997;Stewart et al, 1999). This catastrophe is caused by the continuation of cellular processes in the presence of DNA damage.…”

“…To investigate the function of S-phase checkpoint activation in HU-mediated S-phase arrest, we intentionally used suboptimal HU doses to avoid complete DNA synthesis inhibition. At the highest dose used (1 mM (Stewart et al, 1999;Falck et al, 2002). Interestingly, addition of U0126 to HU-treated cells increased DNA synthesis by 15-20% (data not shown).…”

“…Disruption of DNA damage checkpoints, such as those caused by mutations in ATM, NBS (Nijmegen breakage syndrome), and MRE11, sensitizes cells to genotoxic stresses (Shiloh, 1997;Stewart et al, 1999). To determine whether inhibition of Erk activation enhances cells to HU-mediated toxicity, NIH3T3 cells were treated with HU at 100 mM with or without U0126 (50 mM) for 12 h and then were released into normal medium for approximately 2 weeks.…”

“…Radio-resistant DNA synthesis (RDS) assay RDS assay was carried out following published methodology (Stewart et al, 1999;Falck et al, 2002). Cells were labelled with [ 14 C] thymidine (20 nCi/ml) for 24 h and released into normal medium for additional 24 h. Cells were then pretreated with U0126 for 30 min before addition of HU for 24 h in [ 3 H] thymidine (2.5 mCi/ml) medium.…”

DNA double-strand break repair and development

“…Hypomorphic mutations in Mre11 have been discovered in individuals exhibiting milder characteristics of A-T, resulting in A-T-like disease (ATLD) also characterized by neurodegeneration (Stewart et al, 1999). Likewise, hypomorphic mutations in NBS1 have been identified as the culprit in NBS (Carney et al, 1998;Varon et al, 1998).…”